Temperature probe



Dec. 22, 1964 w, CONQW ETAL 3,162,046

TEMPERATURE PROBE Original Filed Aug. 12, 1960 E E g? v20 50 T 10 r T.

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United States Patent ()fifice 3 Claims. (Cl. 73362) This invention is concerned with means for sensing ram temperature, as is required, for example, in mechanism for computing the true relative density of the atmosphere surrounding an aircraft or other moving vehicle. This application is a division of our copending application Serial No. 49,182, filed August 12, 1960 and entitled Compensated Air Density Computer.

True air density D is related to static pressure P and true air temperature f'by the relation:

D P, T

s0 T where the subscript 0 denotes the value of the indicated quantity under standard conditions of temperature and pressure.

However, it is not possible in a rapidly moving vehicle to measure true static pressure and true air temperature directly. The indicated static pressure P obtained from a conventional static pressure orifice, and the bulb temperature T obtained from a conventional temperature probe both involve deviations which vary with the Mach number of the vehicle.

A primary general object of the invention claimed in the above identified copending application is to provide particularly simple, reliable and economical mechanism for compensating the described deviations and providing a substantially accurate indication of the true air density.

A more particular object of that invention is to provide a computation system in which corrections for bulb temperature and for indicated static pressure are introduced by a single servo loop, thereby greatly reducing the total number of components required in the system.

The present invention provides particularly effective means for sensing bulb temperature. Conventional temperature responsive resistive elements that have a positive temperature coefiicient depart significantly from linearity of response. We have discovered that it is possible to connect two such elements in a passive network in such a way that the output is not only substantially linear out also varies with temperature more rapidly than has previously been possible without the use of active elements.

A full understanding of the invention and of its further objects and advantages will be had from the following description of an illustrative preferred manner of carrying it out. The particulars of that description, and of the drawings which form a part of it, are intended only as illustrative, and not as a limitation upon the scope of the invention, which is defined in the appended claims.

In the drawings:

FIG. 1 is a schematic block diagram representing an illustrative system in accordance with the invention; and

FIG. 2 is a schematic diagram showing further details of the illustrative system of FIG. 1.

In the block diagram of FIG. 1, the potentiometer P1 is supplied with a suitable reference voltage E. The potentiometer wiper is driven by mechanism indicated schematically at in response to indicated static pressure. The output on line 12 is then proportional to P After suitable impedance reduction at 14, the corresponding signal on line 16 is multiplied by a correction factor artists Patented Dec. 22, 1964 (M), which is a definite function of Mach number M. As represented, the signal on line 16 is supplied as reference voltage to the potentiometer PS, the potentiometer wiper being driven in accordance with Mach number via a suitable linkage indicated schematically at 19. That drive may utilize a Mach number computer 29 such as is ordinarily required also for other purposes in an air data computing system. The winding of potentiometer P5 is typically shaped in a known manner to provide the desired function (M), to be described. The voltage tapped from potentiometer P5 on the line 18 then represents the quantity (M)P A voltage representing indicated bulb temperature T which includes the deviation due to ram temperature rise, is developed on the line 32 by mechanism represented schematically at 30. That mechanism may be of conventional type, but preferably comprises the improved multiple unit temperature probe to be described. The voltage on line 32 is supplied as reference voltage to the servodriven balance potentiometer P6. The voltage tapped from P6 on the line 34 is compared with that on line 18, already described, by the differential device indicated schematically at 36. The resulting difference voltage, if any, is supplied via the line 38 to the servo amplifier A1. The amplifier output controls the servo motor Ml, driving potentiometer P6 via the linkage indicated at 39 in such a way as to maintain the difference voltage on line 38 substantially equal to zero. The resulting balance condition of the servo system can be expressed by the equation I where X represents the transfer function of potenti0m eter P6.

In accordance with one aspect of the invention claimed X T sif in the above identified copending application, the single function f(M), introduced by'the correction potentiometer P5, incorporates both the required correction for converting indicated static pressure P to true static pressure P and the correction for converting the indicated or bulb temperature T to true air temperature T. The entire computation of compensated air density may therefore be carried out with only a single servo loop.

For that purpose, potentiometer P5 is so constructed and driven from Mach computer 24) that the correction function f(M) is of the form where r is the recovery factor of the temperature bulb, which is typically equal to 0.85 for a conventional flush mounted temperature bulb. Since r is essentially constant, f (M) is a function of Mach number only, as indicated by the notation.

Pressure correction ratio f (M) represents the error commonly known as the static pressure defect or static source position error. For common static systems the static defect is known'to be a function of Mach number only, and can readily be determined experimentally by known procedures. Over the range of Mach numbers typically encountered in subsonic flight the total variation of f (M) is approximately 10%. Potentiometer P5 can therefore readily be shaped by known methods to provide the required overall function defined in (3) sparing Equations 1 and 5, the transfer function X valance potentiometer P6 is seen to correspond to the sity ratio D/D An output device of any suitable can therefore be coupled to the servo drive 39, as 'esentped at 4-0 in FIG. 2, to provide a signal of de- 1 type representing D. IG. 2 represents an illustrative practical system for ying out the invention, wherein generally correspondparts are numbered as in FIG. 1. Although the ioulars of Mach number computer indicated at 20 1G. 1 are not, in themselves, a part of the present in- :ion, such a computer is included in illustrative form *IG. 2 for clarity of description of the overall con- Pressure responsive elements for controlling Mach .puter 26) are incorporated with static pressure trans- :r 10 of FIG. 1 in the transducer assembly 10a of 2. .s shown illustratively in FIG, 2, transducer assembly comprises the static pressure transducer 50 and the :renti'al pressure transducer 60. Transducer 5i}. comas two electrically independent potentiometers P1 and which are supplied with an alternating current refervoltage E via the lines 51 and 52, respectively, from itable source indicated schematically at 53. The poiometer wipers are driven via suitable coupling means )3! the evacuated capsule 55. The exterior of capsule s exposed to indicated static pressure supplied to the rior of the housing 56 via the conduit 57 from the .c orifice 58. That pressure P differs from the true .0 pressure P by a definite static defect factor, which, already explained, is a function of Mach number, output line from potentiometer P1 thus carries a voltproportional to P and corresponds to line 12 of r. 1. Trimming resistors TR7 and TR3 are preferably nected in series with the winding of P1 to facilitate lstment of the system and promote interchangeability he components. iflterential pressure transducer 60 comprises a potenleter P3, driven via the linkage 61 by a pressure sule 62. Capsule 62 is driven in any suitable manin response to the difference between total pressure and indicated static pressure P For example, in structure shown, the interior of capsule 62. is sup- :l with ram or total pressure P, via the conduit 63 n a suitable total pressure orifice indicated at 64. exterior of capsule 62 is exposed to indicated static :sure P supplied to the interior of the housing 66 n orifice 53 via conduits 57 and 67. The wiper of :ntiometer P3 is therefore moved in proportion to the cated differential pressure q ,=P -P or Mach number computation potentiometers P2 and are connected as rheostats in series between reference age line 52 and ground. The trimming resistor TRl the trimming potentiometer TR2 are connected in zllel between P2 and P3; The voltage tapped from l on the line 68 is a function of the pressure ratio re K is the ratio of the resistance per. unit pressure otentiometer P3 to that in P2. With suitable selecof K, the pressure ratio ('6) can be shown to proa nearly linear measure or" Mach number M over any :h range that is ordinarily required. The small static ct errors in q,, and P are compensated by shaping :ntiometer P4, to be described. Particularly when lble of appreciable length is required between prestransducer unit 19a and other portions of the sysit is desirable to provide impedance isolation for the outputs from lines 12 and 68, as indicated at 14 for line 12 in FIG. 1. The autotransformers T1 and T2 in FIG. 2 typically provide approximately :1 voltage reduction, or 100:1 impedance reduction. The source impedance as viewed by the capacity of output lines 16 and 70 in a shielded cable is thereby reduced to an essentially negligible value, even for a cable as long as 200 feet, for example.

The voltage on line 79, representing pressure ratio ('6), is compared by the summing transformer T3 to the voltage developed at the wiper of the balance potentiometer P4 in Mach computer 2%). The winding of P4 is supplied With reference voltage from source 53 via the voltage dropping resistor R1, and is provided with series connected trimmers TR3 and TR4 which adjust the end points. The error voltage, as derived from the secondary of summing transformer T3 is supplied as input to servo amplifier A2; The amplifier output controls the servomotor 72 and feedback tachometer 73, driving the wiper of balance potentiometer P4 via the gear reduction 74 and the linkage 75. The winding of P4 is suitably shaped to compensate the slight lack of linearity between the signal on line 76, representing pressure ratio (6), and the corresponding Mach number M. The movement of drive 75 thus represents M directly, and may be arranged to drive any desired type of Mach number output device, such as the output potentiometer P7, for example. The servo drive 75 is coupled via linkage 19 to the wiper of potentiometer P5 of the air density computer, and drives it in accordance with Mach number, as already described in connection with FIG. 1.

Turning now more specifically to the air density com putation circuit as shown in FIG. 2, the signal on line 16 representing P is supplied via dropping resistor R2 to the winding of potentiometer P5, which is preferably provided with appropriate end set trimming resistors TRS and TR6. The Winding of potentiometer P5 is Shaped to the correction function f (M), already described. The voltage developed on line 18 by the wiper of P5, driven in accordance with Mach number, is proportional to the product of P and that correction function. That voltage is supplied to one end of the primary of summing transformer T4, which corresponds to dilferential device 36 of FIG. 1.

The other end of the primary of transformer T4 receives on line 34 the voltage tapped from servo balance potentiometer P6. The winding of P6 is provided with the trimming resistor TR9, and is supplied via line 32 with a voltage representing bulb temperature, as already de scribed. The error voltage developed by the secondary of T4 is supplied as input to servo amplifier A1. The servo loop typically includes the motor 82, tachometer 83 and gear reduction 84, and drives the wiper of P6 via linkage 39. As already explained, the wiper movement corresponds directly to the true air density ratio D/D Hence any desired linear output device, such as the output potentiometer P3, may be coupled to linkage 3Q to provide an air density output signal of desired type.

In accordance with the present invention, temperature transducer 30, as shown in FIG. 2, comprises a plurality of temperature responsive resistive elements, shown for illustration as the two elements R5 and R6, which are typically pure nickel temperature bulbs of conventional form. Elements R5 and R6 are mounted in a suitable housing 86 exposed to ram air in a manner corresponding to the usual mounting of a single temperature bulb. The two elements are thus both at the same ram air temperature. They are connected with other passive impedance elements, represented by the resistances R3 and R4, to form an electrical network of any desired configura tion. That network is supplied with the reference volt-. age E via the line 34. In the preferred network shown, resistances R3 and R5 may be considered to form a voltage divider for the reference voltage E, with R4 and R6 series connected in shunt to R5 and forming a voltage.

divider for the voltage signal developed at the junction of R3 and R5. The output T on line 32 is taken from the junction of R4 and R6 and may be expressed as a product of E by two functions representing the actions of the respective voltage dividers:

where the dependence of RS and R6 upon T has been indicated explicitly for clarity.

We have discovered that use of a plurality of temperature bulbs connected in a suitable passive network, of which that shown is illustrative, permits two specific short-comings of previous temperature transducers to be simultaneously corrected. Moreover, that is accomplished without reliance upon active elements, such, for example as amplifying circuits or additional servo loops.

The temperature response of conventional resistive elements with positive temperature coefficient, such as pure nickel, is typically not as large as is desirable, particularly for the present computing system. Moreover, the resistance of such elements increases a little faster than linearly with temperature, approximately in accordance with the formula The present invention provides improved overall response that difiers typically from the form (8) both by making the coemcient B substantially zero, so that the response is essentially linear; and by simultaneously increasing the value of the coefficient A. By suitable selection of component values, the value of A may be increased by nearly a factor of two if linearity of response is not required. For illustrative purpose represented by the overall system of FIGS. 1' and 2, it is preferred to make the response linear and accept a smaller increase in slope. That may typically be done by selecting the values of R3 and R4 substantially equal to 0.37 and 3.7, respectively, times the average value of R5 and R6 over the temperature range of operation. For example, if the resistance of temperature elements R5 and R6 varies from 68 to 94 ohms over the operating temperature range, as is true of typical nickel elements, excellent performance is obtained with R3 and R4 equal to approximately 30 ohms and 300 ohms, respectively.

In the operation of the illustrative system of FIG. 2, the voltage on line 18 represents the product of P by the correction factor defined by Equation 3. That correction factor may be considered as the ratio of two correction factors, one representing the correction needed to convert indicated static pressure to true static pressure, and the other representing the correction needed to convert indicated, or bulb temperature to true air temperature. By introducing both of those correction elements Cir at the same component, namely the winding of poten tiometer PS, the single servo loop containig amplifie A1 can operate directly on the uncorrected indicated tem perature signal on line 32. Yet the output transduce P8, typically driven directly from that loop, represents value of computed air density that is based on correcter temperature as well as corrected static pressure.

We claim:

1. In a temperature probe for sensing atmospheric ran temperature, the combination of two temperature respon sive resistive elements adapted to be exposed toram tern perature, two impedance means, a source of reference volt age, circuit means connecting one impedance means an one element in series with the voltage source, circui means series connecting the other impedance means an the other element in shunt to said one element, and outpr means responsive to the votlage at the junction of sai other impedance means and said other element.

2. In a temperature probe for sensing atmospheri ram temperature, the combination of two temperatur responsive resistive elements adapted to be exposed t rarn temperature, a source of reference voltage, fir: circuit means including one element and acting to deriv a first signal voltage from said reference voltage, secon circuit means including the other element and acting t derive from said first signal voltage a second signal vol age that varies approximately linearly with ram temper: ture, and output circuit means responsive to the secon signal voltage.

3. In a temperature probe for sensing atmospheri ram temperature, the combination of two temperatur responsive resistive elements adapted to be exposed t ram temperature and having approximately equal averag impedance values, first and second impedance meat having impedance values approximately equal, respe tively, to one third and three times said average impedanc values, a source of reference votlage, circuit means cor necting the first impedance means and one element i series with the voltage source, circuit means series cor necting the second impedance means and the other el ment in shunt to said one element, and output mear responsive to the voltage at the junction of said othr impedance means and said other element, the'last sai voltage varying approximately linearly with ram ten perature.

References Cited in the file of this patent UNITED STATES PATENTS 2,533,286 Schmitt Dec. 12, 2,567,755 Amsler Sept. 11, 195 2,649,012 Schnelle Aug. 18, 195 2,862,176 Lustig Nov. 25, 195 2,889,988 Toth et al. June 9, 195 

1. IN A TEMPERATURE PROBE FOR SENSING ATMOSPHERIC RAM TEMPERATURE, THE COMBINATION OF TWO TEMPERATURE RESPONSIVE RESISTIVE ELEMENTS ADAPTED TO BE EXPOSED TO RAM TEMPERATURE, TWO IMPEDANCE MEANS, A SOURCE OF REFERENCE VOLTAGE, CIRCUIT MEANS CONNECTING ONE IMPEDANCE MEANS AND ONE ELEMENT IN SERIES WITH THE VOLTAGE SOURCE, CIRCUIT MEANS SERIES CONNECTING THE OTHER IMPEDANCE MEANS AND THE OTHER ELEMENT IN SHUNT TO SAID ONE ELEMENT, AND OUTPUT MEANS RESPONSIVE TO THE VOLTAGE AT THE JUNCTION OF SAID OTHER IMPEDANCE MEANS AND SAID OTHER ELEMENT. 